core: add new function to allow a layer to add other layers
core: add new function to allow a layer to add other layers

- configuration-layer/declare-layers
- configuration-layer/declare-layer

These functions can be used in the config.el file of a layer to add
required layers.

For instance the react layer needs both html and javascript layers.
salt layer needs the yaml layer (see the following commits)

tsonntag/dotemacs,tsonntag/dotemacs,tsonntag/dotemacs,tsonntag/dotemacs